This invention relates to a pressure sensor and, more particularly, to a pressure sensor for use in an automotive internal combustion engine in which a pressure transmitting liquid is filled between a pressure detection assembly and a flexible diaphragm.
FIG. 2 illustrates one example of a conventional pressure sensor for use in detecting gas pressure within a combustion chamber of an internal combustion engine. In FIG. 2, reference numeral 1 designates a stainless steel case to be attached to an engine (not shown), 2 designates a diaphragm attached to an inner or lower end of the case 1, 3 designates an electrically insulating base, 4 designates a mounting bed made of a ceramic material and welded to the case 1, and 5 designates a pressure sensing element bonded to the mounting bed 4. The pressure sensing element 5 illustrated is a silicon semiconductor strain gauge. A pressure transmitting liquid of silicone oil 10 having a high boiling point is filled between the pressure sensing element 5 and the flexible diaphragm 2.
Reference numeral 6 designates terminals attached to the base 3, 7 designates bonding wires connecting the terminals 6 to the pressure sensing element 5, 8 designates a plug thread-engaged with the case 1, and 9 designates output lines extending through the plug 8 and connected at one end to the terminals 6.
The pressure increase within the combustion chamber of an internal combustion engine causes the diaphragm 2 to flex upward in FIG. 2 and the pressure of the silicone oil 10 is increased, which in turn is sensed by the pressure sensing element 5. Then the pressure sensing element 5 generates an electrical signal indicative of the pressure change and supplies it through the bonding wires 7, the terminals 6 and the output lines 9 to an unillustrated external circuit for controlling the ignition timing of the ignition plug in accordance with the combustion conditions within the combustion chamber.
During assembly of the conventional pressure sensor as described above, the base 3 on which the terminals 6 are attached and the mounting bed 4 on which the pressure sensing element 5 is bonded are securely attached to the inner surfaces of the substantially cup-shaped case 1. Thereafter, the pressure sensing element 5 and the terminals 6 are wire-bonded within the cup-shaped casing 1 to electrically connect them by the bonding wires 7. Therefore, the wire-bonding operation must be carried out within the relatively narrow and hard-to-access space defined in the case 1, making the assembling efficiency relatively poor.
With the conventional pressure sensor, the test and calibration of the pressure sensing element can be carried out only after the above assembly including welding and wire-bonding operations has been completed. Therefore, sometimes the assembled pressure sensing element is found to be non-conforming only after the assembly, wasting time and materials used in assembly.
Also, the pressure transmitting liquid must be filled within the cavity formed in the main body when the mounting bed 4 with the pressure sensing element thereon or the flexible diaphragm is being attached to the main body. Also, since the mounting bed 4 supporting the pressure sensing element 5 is welded to the case 1, the welding heat applied during the assembly causes the mounting bed 4 to distort and deform, which results in an undesirable effect on the pressure sensing element 5 which may result in detection errors.